# std::ranges::next

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
Call signature |  |
template< std::input_or_output_iterator I > constexpr I next( I i ); | (1) | (since C++20)
template< std::input_or_output_iterator I > constexpr I next( I i, std::iter_difference_t<I> n ); | (2) | (since C++20)
template< std::input_or_output_iterator I, std::sentinel_for<I> S > constexpr I next( I i, S bound ); | (3) | (since C++20)
template< std::input_or_output_iterator I, std::sentinel_for<I> S > constexpr I next( I i, std::iter_difference_t<I> n, S bound ); | (4) | (since C++20)

Return the nth successor of iterator i.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

i | - | an iterator
n | - | number of elements to advance
bound | - | sentinel denoting the end of the range i points to

### Return value

1) The successor of iterator i.
2) The nth successor of iterator i.
3) The first iterator equivalent to bound.
4) The nth successor of iterator i, or the first iterator equivalent to bound, whichever is first.

### Complexity

1) Constant.
2) Constant if `I` models std::random_access_iterator; otherwise linear.
3) Constant if `I` and `S` models both std::random_access_iterator<I> and std::sized_sentinel_for<S, I>, or if `I` and `S` models std::assignable_from<I&, S>; otherwise linear.
4) Constant if `I` and `S` models both std::random_access_iterator<I> and std::sized_sentinel_for<S, I>; otherwise linear.

### Possible implementation

    struct next_fn
    {
        template<std::input_or_output_iterator I>
        constexpr I operator()(I i) const
        {
            ++i;
            return i;
        }

        template<std::input_or_output_iterator I>
        constexpr I operator()(I i, std::iter_difference_t<I> n) const
        {
            ranges::advance(i, n);
            return i;
        }

        template<std::input_or_output_iterator I, std::sentinel_for<I> S>
        constexpr I operator()(I i, S bound) const
        {
            ranges::advance(i, bound);
            return i;
        }

        template<std::input_or_output_iterator I, std::sentinel_for<I> S>
        constexpr I operator()(I i, std::iter_difference_t<I> n, S bound) const
        {
            ranges::advance(i, n, bound);
            return i;
        }
    };

    inline constexpr auto next = next_fn();

### Notes

Although the expression ++x.begin() often compiles, it is not guaranteed to do so: x.begin() is an rvalue expression, and there is no requirement that specifies that increment of an rvalue is guaranteed to work. In particular, when iterators are implemented as pointers or its `operator++` is lvalue-ref-qualified, ++x.begin() does not compile, while ranges::next(x.begin()) does.

### Example

Run this code

    #include <cassert>
    #include <iterator>

    int main()
    {
        auto v = {3, 1, 4};
        {
            auto n = std::ranges::next(v.begin());
            assert(*n == 1);
        }
        {
            auto n = std::ranges::next(v.begin(), 2);
            assert(*n == 4);
        }
        {
            auto n = std::ranges::next(v.begin(), v.end());
            assert(n == v.end());
        }
        {
            auto n = std::ranges::next(v.begin(), 42, v.end());
            assert(n == v.end());
        }
    }

### See also

ranges::prev (C++20) | decrement an iterator by a given distance or to a bound (algorithm function object)
ranges::advance (C++20) | advances an iterator by given distance or to a given bound (algorithm function object)
next (C++11) | increment an iterator (function template)

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